docs(task): archive secret lifecycle remediation

[blakinio]

Purpose

Close the lifecycle of completed task OTC2-20260801-secret-lifecycle-remediation after implementation PR #124 merged.

Merge evidence

• final restacked implementation head: 821bd5871e708e7d03b3d375bbad537ff40ef11a;
• implementation squash merge: c6d11a6c26f75c2169913e297c14b0ec25419736;
• Rust Client run 30690135419: Windows workspace and Supply Chain passed;
• repository CI run 30690135509: passed including CI / Required;
• final review gate: no comments, reviews or unresolved threads.

Lifecycle change

• add the completed record under docs/agents/tasks/archive/;
• remove the corresponding record from docs/agents/tasks/active/;
• preserve the exact R1 security boundary and defer R2 until this archive PR merges.

Scope

Exactly two task-lifecycle Markdown paths. No Rust, manifest, lockfile, workflow, policy, architecture, test or external-repository change.

Next authorized step

After this PR merges, perform a fresh main/PR/task/path/contract/shared-lease preflight for OTC2-AUD-002.